Still image and video cameras are ubiquitous in today's society. Almost all mobile telephones include an integrated camera, and consumers often select one mobile telephone over another based on the quality of the integrated camera. In addition, portable point and shoot, video, and multi-function cameras are also common. Single lens reflex (SLR) cameras have become commonplace for users that desire to enhance image capture speed and quality.
Features that are desirable in any camera include a high dynamic range such that bright regions in captured images do not appear washed out and dark regions do not lose detail. Another desirable camera feature is the ability to select regions of interest in a captured image and to selectively apply higher dynamic range or other image processing enhancement to the region of interest. In video cameras, higher or variable frame rates may be desirable. It is also desirable to be able to dynamically trade off one feature, such as dynamic range, for another feature, such as frame rate. Another camera feature that may be desirable is frameless image capture, where the exposure time for the individual photodetectors (pixels) does not depend on a predefined start and stop time.
These often competing goals are not achievable by today's cameras. For example, the integrators that accumulate the charge output by pixels on camera phones are typically small capacitors that are easily saturated. Once saturated, the resultant image appears white or washed out. Increasing the integration rate may yield higher quality in darker images but quicker saturation for lighter images. Using a larger integrator that takes longer to saturate may lead to lower quality in dark regions.
Additional areas for improvement for existing cameras include in-pixel processing and column readout. In-pixel processing refers to on-chip processing of the output from the integrator and typically involves an analog to digital converter (ADC). Column readout refers to communication signals for a column of pixels from the photodetector array to post-processing circuitry for image reconstruction. It is possible to convert the signal output from the integrator to digital signal prior to column readout. However, analog to digital conversion prior to readout requires an ADC adjacent to each pixel, which reduces the amount of chip area for the photodiodes. Another option is to send analog values to the post processing circuitry. This solution takes up less area in the photodetector array, but analog signals are susceptible to degradation during transmission (because of noise and crosstalk, for example).
Thus, in light of these difficulties, there exists a need for an improved camera sensor that achieves at least some of the goals described above.